The present invention relates to a shift control method for a sub-transmission incorporated into the power transmission system of a vehicle such as an automobile, and more particularly relates to a shift control method for performing properly the shifting of such a sub-transmission in accordance with the operating conditions of the vehicle, bearing in mind the style of driving and of gear shifting of the driver.
It is known to incorporate a two-speed sub-transmission, which can be shifted between a high speed stage and a low speed stage, into the power train of a vehicle such as an automobile which also is equipped with a manual transmission.
The control of such a sub-transmission has sometimes been manual; however, this puts a strain on the driver, and distracts him unduly from the business of vehicle control. As a result, such manual control has not been practical from the point of view of obtaining the proper shift timing of the sub-transmission to ensure good performance and fuel economy.
In order to avoid this problem, it has been proposed to control such a sub-transmission electrically or electronically, by shifting it over from the low speed stage to the high speed stage, and from the high speed stage to the low speed stage, automatically, depending upon combinations of various vehicle and/or engine operating parameters, such as the vehicle speed, the engine revolution speed, the throttle opening, the engine load, etc. Typically, the shifting is performed in accordance with a transmission diagram, which is a line on a graph of a signal representing either vehicle speed or engine speed, against a signal representing engine load, such as the throttle opening.
Now, the shifting of the manual transmission is performed by the driver, according to his own free will, and the precise shift points will vary according to the driver's habits, the conditions of the road, and so on. If, therefore, the transmission diagram of the sub-transmission is determined independently of these conditions, and shifting of the sub-transmission is performed in accordance therewith, it may occur that this shifting is performed at times which rather go against the wish of the driver, as expressed by his particular current pattern of gear shifting.
For instance, if the driver currently does not perform upshifting until the speed of the vehicle has become relatively high, so that he is performing so-called high-revving or high acceleration operation, it is desirable that the sub-transmission, responding to this mode of operation, likewise should not shift from its low speed stage to its high speed stage until the vehicle speed has become relatively high. Thus the driving style of the driver and the sub-transmission will blend harmoniously.
If, on the other hand, the sub-transmission does not respond to this high-revving mode of operation by the driver, then the shift of the sub-transmission from its low speed stage to its high speed stage may occur almost immediately after the manual upshift by the driver, so that, in effect, high-revving operation is not performed, and the driver's intentions are frustrated.
Similarly if, on the other hand, the driver is currently in the habit of performing upshifting rather early, so that the vehicle is being operated in a low-revving or sedate mode, (which provides good fuel economy), then, again, if the automatic control of the sub-transmission does not respond to this mode of manual shifting, a rather long time will elapse after upshifting by the driver before the sub-transmission shifts from its low speed stage to its high speed stage, and thus low-revving operation of the vehicle will not take place, and again the driver's intentions will be frustrated.
Therefore, in a co-pending patent application Ser. No. 060,003, it has been proposed to perform the control of a sub-transmission of a vehicle which is fitted with a manual transmission in a manner which depends upon the style of driving of the vehicle operator.
However, there is a point in this proposed method of sub-transmission shift control which induces a further development of this basic concept of sub-transmission shift control. If the driver of the vehicle performs a change to a certain transmission ratio, and thereafter operates the vehicle for a considerable time in that ratio, as for example during open road driving, the operation of the sub-transmission will be determined for a very considerable time on the somewhat fortuitous basis of the exact speed at which gear shifting to that ratio was last made. This is not generally logical, and furthermore may well not be what the driver desires.
From the point of view of fuel economy, the transmission ratio being fixed, there is an optimum line on a chart of engine rotational speed versus torque for operating a vehicle. If the engine is operated along that line, maximum fuel economy will result. In order to operate the engine as close along that line as possible, under the given conditions of the vehicle speed and the required output power, the engine rotational speed may be controlled more desirably by shifting the sub-transmission of the vehicle. Of course, the engine cannot thereby be operated exactly along that line, but an approximation can be made thereto. Therefore, from the point of view of maximizing fuel economy, there is an optimum manner of operating the sub-transmission.
It has occurred to the present inventors that it would be ideal to perform the first, or operator-sensitive, sub-transmission control, until a steady vehicle operation commences, and thereafter to perform the second, or fuel-economical, sub-transmission control.